Many gas turbine engines use a heat exchanger in the form of a primary surface recuperator to increase the operating efficiency of the engine by extracting heat from the exhaust gas and preheating the intake air. Typically, a recuperator for a gas turbine engine must be capable of operating at temperatures of about 650.degree. C. and internal pressures of approximately 550 kPa under operating conditions involving repeated starting and stopping cycles. In some large turbine engine installations, the recuperator may be 3 meters or longer.
Such recuperators include a core which is commonly constructed from a plurality of stacked side-by-side thin stainless steel sheets. Successive pairs of the sheets are joined at their periphery to form passages called air cells. Compressed discharged air from a compressor of the engine passes through the air cells while the hot exhaust gas flows through the passages formed by the exterior surfaces of each adjacent pair of air cells. The exhaust gas heats the sheets and the intake air from the compressor absorbs the heat from the sheets. Support for the air cells is provided by clamping the stack of air cells, commonly called a core, between two rigid end beams. Such end beams prevent the air cells from "ballooning" due to internal pressure of the intake air. The clamping force heretofore has been provided by either external or internal restraint systems which rigidly interconnect the two end beams.
An example of an external restraint system is disclosed in U.S. Pat. No. 4,090,358 issued to D. Craig Young on May 23, 1978. In such system, the restraining members are located externally of the recuperator. One of the problems with such restraint system is the drastically different thermal response time of the restraint members as compared to the thermal response time of the core. For example, when the engine is started, the exhaust gas and recuperator heat very rapidly causing the core to grow rapidly due to thermal expansion of the components. Since the restraining members are located externally of the recuperator, they are not heated as rapidly as the core and the rate of thermal expansion thereof is much slower than the expansion rate of the core. This thermal growth difference causes a thermal tension load on the restraining members and a compressive load on the recuperator in addition to the load from internal air pressure. These combined loads can exceed the compressive strength of the recuperator causing it to yield to a compressed length. When the recuperator and restraining members reach thermal stability, the compressed recuperator is no longer supported by the restraint system and the recuperator internal structure is subjected to the force caused by the internal air pressure. This overloading of the recuperator structure by the internal air pressure can result in reduced low cycle fatigue life. Low cycle fatigue causes cracking in the air cells adjacent each end of the core allowing air to leak therefrom which thereby reduces the efficiency of the recuperator.
An example of an internal restraint system is disclosed in U.S. Pat. No. 4,331,352 issued to Richard F. Graves on May 25, 1982. That disclosure utilizes a plurality of independent, large diameter tie rods which extend through the exhaust gas flow path and between flanges at opposite ends of the recuperator. That patent recognizes that the tie rods and core experience thermal growth and consequently separate additional devices were provided to accommodate such thermal growth. Such additional devices add to the complexity of constructing the recuperator and add additional cost thereto.
The present invention is directed to overcome one or more of the problems as set forth above.